Throughout this disclosure, various publications are referenced by first author and date, patent number or publication number. The full bibliographic citation for each reference can be found at the end of this application, immediately preceding the claims. The disclosures of these references are hereby incorporated by reference into this disclosure to more fully describe the state of the art to which this invention pertains.
Cancer cells are characterized by uncontrolled growth, de-differentiation and genetic instability. The instability expresses itself as aberrant chromosome number, chromosome deletions, rearrangements, loss or duplication beyond the normal dipoid number. Wilson, J. D. et al. (1991). This genomic instability may be caused by several factors. One of the best characterized is the enhanced genomic plasticity which occurs upon loss of tumor suppression gene function (e.g., Almasan, A. et al. (1995)). The genomic plasticity lends itself to adaptability of tumor cells to their changing environment, and may allow for the more frequent mutation, amplification of genes, and the formation of extrachromosomal elements (Smith, K. A. et al. (1995) and Wilson, J. D. et al. (1991)). These characteristics provide for mechanisms resulting in more aggressive malignancy because it allows the tumors to rapidly develop resistance to natural host defense mechanisms, biologic therapies (Wilson, J. D. et al. (1991) and Shepard, H. M. et al. (1988)), as well as to chemotherapeutics. Almasan, A. et al. (1995) and Wilson, J. D. et al. (1991).
Cancer is one of the most commonly fatal human diseases worldwide. Treatment with anticancer drugs is an option of steadily increasing importance, especially for systemic malignancies or for metastatic cancers which have passed the state of surgical curability. Unfortunately, the subset of human cancer types that are amenable to curative treatment today is still rather small (Haskell, C. M. eds. (1995), p. 32). Progress in the development of drugs that can cure human cancer is slow. The heterogeneity of malignant tumors with respect to their genetics, biology and biochemistry as well as primary or treatment-induced resistance to therapy mitigate against curative treatment. Moreover, many anticancer drugs display only a low degree of selectivity, causing often severe or even life threatening toxic side effects, thus preventing the application of doses high enough to kill all cancer cells. Searching for anti-neoplastic agents with improved selectivity to treatment-resistant pathological, malignant cells remains therefore a central task for drug development. In addition, widespread resistance to antibiotics is becoming an important, world-wide, health issue. Segovia, M. (1994) and Snydman, D. R. et al. (1996).
Classes of Chemotherapeutic Agents
The major classes of agents include the alkylating agents, antitumor antibiotics, plant alkaloids, antimetabolites, hormonal agonists and antagonists, and a variety of miscellaneous agents. See Haskell, C. M., ed., (1995) and Dorr, R. T. and Von Hoff, D. D., eds. (1994).
The classic alkylating agents are highly reactive compounds that have the ability to substitute alkyl groups for the hydrogen atoms of certain organic compounds. Alkylation of nucleic acids, primarily DNA, is the critical cytotoxic action for most of these compounds. The damage they cause interferes with DNA replication and RNA transcription. The classic alkylating agents include mechlorethamine, chlorambucil, melphalan, cyclophosphamide, ifosfamide, thiotepa and busulfan. A number of nonclassic alkylating agents also damage DNA and proteins, but through diverse and complex mechanisms, such as methylation or chloroethylation, that differ from the classic alkylators. The nonclassic alkylating agents include dacarbazine, carmustine, lomustine, cisplatin, carboplatin, procarbazine and altretamine.
The clinically useful antitumor drugs are natural products of various strains of the soil fungus Streptomyces. They produce their tumoricidal effects by one or more mechanisms. All of the antibiotics are capable of binding DNA, usually by intercalation, with subsequent unwinding of the helix. This distortion impairs the ability of the DNA to serve as a template for DNA synthesis, RNA synthesis, or both. These drugs may also damage DNA by the formation of free radicals and the chelation of important metal ions. They may also act as inhibitors of topoisomerase II, an enzyme critical to cell division. Drugs of this class include doxorubicin (Adriamycin), daunorubicin, idarubicin, mitoxantrone, bleomycin, dactinomycin, mitomycin C, plicamycin and streptozocin.
Plants have provided some of the most useful antineoplastic agents. Three groups of agents from this class are the Vinca alkaloids (vincristine and vinblastine), the epipodophyllotoxins (etoposide and teniposide) and paclitaxel (Taxol). The Vinca alkaloids bind to microtubular proteins found in dividing cells and the nervous system. This binding alters the dynamics of tubulin addition and loss at the ends of mitotic spindles, resulting ultimately in mitotic arrest. Similar proteins make up an important part of nervous tissue; therefore, these agents are neurotoxic. The epipodophyllotoxins inhibit topoisomerase II and therefore have profound effects on cell function. Paclitaxel has complex effects on microtubules.
The antimetabolites are structural analogs of normal metabolites that are required for cell function and replication. They typically work by interacting with cellular enzymes. Among the many antimetabolites that have been developed and clinically tested are methotrexate, 5-fluorouracil (5-FU), floxuridine (FUDR), cytarabine, 6-mercaptopurine (6-MP), 6-thioguanine, deoxycoformycin, fludarabine, 2-chlorodeoxyadenosine, and hydroxyurea.
Endocrine manipulation is an effective therapy for several forms of neoplastic disease. A wide variety of hormones and hormone antagonists have been developed for potential use in oncology. Examples of available hormonal agents are diethylstilbestrol, tamoxifen, megestrol acetate, dexamethasone, prednisone, aminoglutethimide, leuprolide, goserelin, flutamide, and octreotide acetate.
Drawbacks of Current Chemotherapeutic Agents
Among the problems currently associated with the use of chemotherapeutic agents to treat cancers are the high doses of agent required; toxicity toward normal cells, i.e., lack of selectivity; immunosuppression; second malignancies; and drug resistance.
The majority of the agents that are now used in cancer chemotherapy act by an anti-proliferative mechanism. However, most human solid cancers do not have a high proportion of cells that are rapidly proliferating and they are therefore not particularly sensitive to this class of agent. Moreover, most antineoplastic agents have steep dose-response curves. Because of host toxicity, treatment has to be discontinued at dose levels that are well below the dose that would be required to kill all viable tumor cells.
Another side effect associated with present day therapies is the toxic effect of the chemotherapeutic on the normal host tissues that are the most rapidly dividing, such as the bone marrow, gut mucosa and cells of the lymphoid system. The agents also exert a variety of other adverse effects, including neurotoxicity; negative effects on sexuality and gonadal function; and cardiac, pulmonary, pancreatic and hepatic toxicities; vascular and hypersensitivity reactions, and dermatological reactions.
Hematologic toxicity is the most dangerous form of toxicity for many of the antineoplastic drugs used in clinical practice. Its most common form is neutropenia, with an attendant high risk of infection, although thrombocytopenia and bleeding may also occur and be life threatening. Chemotherapy may also induce qualitative defects in the function of both polymorphonuclear leukocytes and platelets. The hematopoietic growth factors have been developed to address these important side effects. Wilson, J. D. et al. (1991) and Dorr, R. T. and Von Hoff, D. D., eds. (1994).
Most of the commonly used antineoplastic agents are capable of suppressing both cellular and humoral immunity. Infections commonly lead to the death of patients with advanced cancer, and impaired immunity may contribute to such deaths. Chronic, delayed immunosuppression may also result from cancer chemotherapy.
The major forms of neurotoxicity are arachnoiditis; myelopathy or encephalomyelopathy; chronic encephalopathies and the somnolence syndrome; acute encephalopathies; peripheral neuropathies; and acute cerebellar syndromes or ataxia.
Many of the commonly employed antineoplastic agents are mutagenic as well as teratogenic. Some, including procarbazine and the alkylating agents, are clearly carcinogenic. This carcinogenic potential is primarily seen as delayed acute leukemia in patients treated with polyfinctional alkylating agents and inhibitors of topoisomerase II, such as etoposide and the anthracycline antibiotics. Chemotherapy has also been associated with cases of delayed non-Hodgkin's lymphoma and solid tumors. The present invention will minimize these effects since the prodrug will only be activated within tumor cells.
The clinical usefulness of a chemotherapeutic agent may be severely limited by the emergence of malignant cells resistant to that drug. A number of cellular mechanisms are probably involved in drug resistance, e.g., altered metabolism of the drugs, impermeability of the cell to the active compound or accelerated drug elimination from the cell, altered specificity of an inhibited enzyme, increased production of a target molecule, increased repair of cytotoxic lesions, or the bypassing of an inhibited reaction by alternative biochemical pathways. In some cases, resistance to one drug may confer resistance to other, biochemically distinct drugs. Amplification of certain genes is involved in resistance to biologic and chemotherapy. Amplification of the gene encoding dihydrofolate reductase is related to resistance to methotrexate, while amplification of the gene encoding thymidylate synthase is related to resistance to treatment with 5-fluoropyrimidines. Table 1 summarizes some prominent enzymes in resistance to biologic and chemotherapy.
TABLE 1Enzymes Overexpressed in Resistance to ChemotherapyBiologic orEnzymeChemotherapyReferenced (Examples)Thymidylate synthaseUracil-basedLönn, U. et al.Folate-basedKobayashi, H. et al.Quinazoline-basedJackman, AL et al.Dihydrofolate reductaseFolate-basedBanerjee, D. et al.Bertino, J. R. et al.Tyrosine kinasesTNF-alphaHudziak, R. M. et al.MultidrugStühlinger, M. et al.resistanceMDR-associated proteinsMultidrugSimon, S. M. and(ABC P-gp proteins)resistanceSchindler, M.Gottesman, M. M. et al.CAD*PALLA**Smith, K. A. et al.Dorr, R. T. andVon Hoff, D. D., eds.Ribonucleotide reductaseHydroxyureaWettergren, Y. et al.Yen, Y. et al.*CAD = carbamyl-P synthase, aspartate transcarbamylase, dihydroorotase**PALA = N-(phosphonacetyl)-L-aspartateUse of Prodrugs as a Solution to Enhance Selectivity of a Chemotherapeutic Agent
The poor selectivity of anticancer agents has been recognized for a long time and attempts to improve selectivity and allow greater doses to be administered have been numerous. One approach has been the development of prodrugs. Prodrugs are compounds that are toxicologically inert but which may be converted in vivo to active toxic products. In some cases, the activation occurs through the action of a non-endogenous enzyme delivered to the target cell by antibody (“ADEPT” or antibody-dependent enzyme prodrug therapy (U.S. Pat. No. 4,975,278)) or gene targeting (“GDEPT” or gene dependent enzyome-prodrug therapy (Melton, R. G. and Sherwood, R. F. (1996)). These technologies have severe limitations with respect to their ability to exit the blood and penetrate tumors. Connors, T. A. and Knox, R. J. (1995).
Accordingly, there is a need for more selective agents which can penetrate the tumor and inhibit the proliferation and/or kill cancer cells that have developed resistance to therapy. The present invention satisfies this need and provides related advantages as well.